Can we limit warming
to 2 ºC?
The AVOID2 programme — led by the Met
Office in a UK consortium with the Walker
Institute, Tyndall Centre and Grantham
Institute — has demonstrated that it is possible
to restrict warming to 2 °C or less during the
21st century with at least a 50% probability.
KEY FINDINGS
• AVOID has shown that limiting global
warming to 2 °C is possible but challenging.
Early global action increases the chances of
remaining below this level.
• For a 90% chance of staying below 2 °C,
AVOID results show that we will probably
need negative emissions and/or some
geo-engineering intervention.
Maximum temperature
during the 21st century
relative to 1750 (°C)
AVOID is a UK research programme
funded by DECC/Defra and led by the
Met Office in a consortium with the
Walker Institute, Tyndall Centre and
Grantham Institute.
There is now international agreement from the G8/G20 and in the
Copenhagen Accord1 that to avoid dangerous climate change,
warming should be limited to 2 °C or less above pre-industrial levels.
Greater warming could lead to potentially intolerable climate
impacts and is more likely to trigger accelerated or irreversible
environmental changes.
3.5
1%
3.0
2%
3%
4%
5%
2.5
2.0
1.5
2015
2020
2025
2030
Emissions peak year
Which emission trajectory yields a temperature that remains
below 2 °C, 3 °C or 4 °C with at least a 50% probability?
The 2 °C target is taken here to mean 2 °C at 2100 relative
to pre-industrial (1750).
• There is some limited flexibility in the year
of peak emissions and the emissions in
2020. The later the peak year, the stronger
the action must be to reduce emissions after
the peak. This may be beyond economic and
technological feasibility.
For example: Should emissions peak in
2016, to limit warming to 2 °C with a 50%
probability will require emissions reductions
of approximately 4% per year. But should
emissions peak later in 2020, to achieve the
same temperature target will require a faster
rate of emissions reductions of almost 6%
per year.
1 The Accord does not specify a
baseline period for temperature,
but scientific analysis typically
assumes this to be pre-industrial.
2 Avoiding Dangerous Climate
Change.
• Emissions after 2050 matter. For instance,
an extra 100 years of carbon dioxide (CO2)
emissions at just a fifth of the year 2000-level
could add more than 0.3 °C to peak
warming.
• How the 2 °C target is defined is a very
important consideration. It has implications
for the level of action required and the
impacts that are avoided by taking action
instead of continuing with business as usual.
2035
WHAT DOES THIS MEAN FOR
SEA-LEVEL RISE?
WHAT IS THE 2 °C TARGET?
Stabilising greenhouse gas concentrations in
the atmosphere can significantly reduce the
rate of warming, levelling it out significantly in
a few decades. Moving from a world heading
towards, say, a 4 °C rise in global average
temperature to one where warming is limited
to a 2 °C rise is an enormous task, but one that
might also avoid up to a third of predicted
21st century sea-level rise. Going from a 4 °C
world to a 2 °C world would also lower the
chance of triggering irreversible melting of
the Greenland ice-sheet, which could
eventually add several metres to sea-levels
across the globe.
The following issues need to be considered:
•Timescale — the target can be defined so
that i) it is met only during 21st century,
but not later; ii) it is met during 21st and
22nd centuries but not later; and iii) it is met
forever (taken as 2500 in our simulations).
40
However, even a very large reduction in
emissions will not avoid all future increases in
temperature or sea-level. In the case of
sea-level, the increases are likely to continue
for several hundred years after greenhouse gas
concentrations are stabilised.
Business as usual
35
Sea-level rise
30
25
(b)
Sea-level rise (m)
Equivalent CO2 emissions (GtC/yr)
•Acceptable level of risk — the UK
Committee on Climate Change worked
with the aim of staying below 2 °C up to
2100 with at least approximately a 50%
probability. The choice of an acceptable level
of risk is a political one, but the science can
provide information on the consequences of
alternative choices.
20
(c)
15
10
5
0
(d)
Mitigation
(a)
2000
2040
2080
2120
2160
2200
0.5
4 °C world
0.4
2 °C world
0.3
0.2
0.1
0
Year
SRES A1B
A1B _2016:5%_low
2000
2020 2040
2060 2080
2100
Year
Mitigation scenario: Key parameters to vary — a) year of
peak emissions; b) rate of pre-peak emission increase;
c) post-peak rate of emission reduction; d) long-term
emissions floor.
The green lines show the response in an unmitigated
world that reaches 4 °C by 2100, compared to a world
with rapid and early mitigation that limits warming to
2 °C by 2100 (purple lines). The thick lines correspond to
the medium model response and the thin lines show the
uncertainty range.
For more information on the scientific content of AVOID, contact:
Dr Jason Lowe, Chief Scientist for AVOID, Met Office Hadley Centre, UK
Email: avoid-chiefsci@metoffice.gov.uk Tel: +44 (0)118 378 5612
For more information on how to join AVOID, contact:
Dr Jolene Cook, Programme Officer for AVOID, DECC, UK
Email: jolene.cook@decc.gsi.gov.uk Tel: +44 (0)300 068 5589
To receive updates on the AVOID programme:
Email: avoiding@metoffice.gov.uk
Met Office
FitzRoy Road, Exeter
Devon, EX1 3PB
United Kingdom
Tel: 0870 900 0100
Fax: 0870 900 5050
enquiries@metoffice.gov.uk
www.metoffice.gov.uk
Produced by the Met Office
© Crown copyright 2010 10/0202b
Met Office and the Met Office logo
are registered trademarks